Bacterial polysaccharides (PSs) are T-independent antigens inducing short-term immunity in older children and adults, but frequently not in young infants. PSs are incapable of binding to the major histocompatibility complex molecules, which is required for antigen presentation to and stimulation of T-helper lymphocytes. PSs are able to stimulate B lymphocytes for antibody production without the help of T-helper lymphocytes. As a result of the T-independent stimulation of the B lymphocytes, there is a lack of memory induction following immunization by these antigens.
T-independent polysaccharide antigens can be converted to T-dependent antigens by covalent attachment of the polysaccharides to protein molecules. B cells that bind the polysaccharide component of the conjugate vaccine can be activated by helper T cells specific for peptides that are a part of the conjugated carrier protein. The T-helper response to the carrier protein serves to augment the antibody production to the polysaccharide. PS-conjugate vaccines are polysaccharide-protein hybrids formed by the covalent attachment of a protein to a PS. Chemical modification of the PS prior to attachment is typically required because most native bacterial PSs cannot be chemically linked to a protein without first undergoing some chemical modification (“activation”).
Attachment to the protein yields a number of T cell epitopes. These T cell epitopes interact with CD4 helper T cells, greatly facilitating an antibody response to the attached polysaccharide. The T helper cell-dependent response to a conjugate results in both serum IgG antibodies and immune memory, even in infants. Additionally, the immunogenicity of the PS-conjugate, in contrast to the native PS, is less dependent on the size of the conjugated PS. Accordingly, conjugates prepared with either PS or oligosaccharides can have similar immunogenicity.
There are many conjugation reactions that have been employed for covalently linking polysaccharides to proteins. Three of the more commonly employed methods include: 1) reductive amination, wherein the aldehyde or ketone group on one component of the reaction reacts with the amino or hydrazide group on the other component, and the C═N double bond formed is subsequently reduced to C—N single bond by a reducing agent; 2) cyanylation conjugation, wherein the polysaccharide is activated either by cyanogens bromide (CNBr) or by 1-cyano-4-dimethylammoniumpyridinium tetrafluoroborate (CDAP) to introduce a cyanate group to the hydroxyl group, which forms a covalent bond to the amino or hydrazide group upon addition of the protein component; and 3) a carbodiimide reaction, wherein carbodiimide activates the carboxyl group on one component of the conjugation reaction, and the activated carbonyl group reacts with the amino or hydrazide group on the other component. These reactions are also frequently employed to activate the components of the conjugate prior to the conjugation reaction.
The Haemophilus influenzae type b (Hib) conjugate vaccines represent the first PS-protein conjugate vaccines produced for clinical use. Robbins and his colleagues in 1980 utilized the biotechnological process of chemically attaching saccharides to protein carriers, a concept developed 50 years earlier. See Avery et al., J. Exp. Med. 1929; 50:533-550; Schneerson et al., J. Exp. Med. 1980; 152:361-376. There are now four different Hib conjugate vaccines licensed in the United States, each different, and each having their own physical, chemical, and immunological characteristics, as summarized in Table 1. A detailed review of the conjugation chemistry and quality control used in these vaccines has been published. See Kniskern et al., “Conjugation: design, chemistry, and analysis” in Ellis et al., Development and clinical uses of Haemophilus b conjugate vaccines. New York: Marcel Dekker, 1994: 37-69.
TABLE 1Vaccine*Saccharide sizeCarrier proteinSpacer (linker)PRP-DPolysaccharideDiphtheria toxoid6-carbon spacer(Connaught)(ADH)HbOCOligosaccharideDiphtheria proteinNone (amide)(Wyeth-Lederle)(CRM)PRP-OMPCSmallMeningococcalThioether(Merck)polysaccharideprotein(bigeneric)PRP-TpolysaccharideTetanus toxoid6-carbon spacer(Aventis Pasteur)(ADH)*The four Hib conjugate vaccines are described commonly in the literature with these acronyms and the responsible manufacturers are in parentheses.
The first commercial Hib conjugate, polyribosylribitol phosphate diphtheria toxoid conjugate (PRP-D), consists of partially size-reduced Hib PS attached through a six-carbon spacer, adipic acid dihydrazide (ADH), to diphtheria toxoid using the procedure of Schneerson et al., J. Exp. Med. 1980; 152:361-376. The ADH derivative of diphtheria toxoid was obtained in this method by reaction with ADH in the presence of 1-[3-(dimethylamino)propyl]-3-ethyl carbodiimide hydrochloride (EDC). The Hib PS was then activated by creating cyanate groups on the hydroxyl groups using CNBr. The activated PS was conjugated to the ADH-toxoid (cyanylation conjugation), but the process created an unstable linkage and the conjugate had solubility problems.
The Robbins conjugation chemistry was later modified such that the ADH spacer is added first to the polysaccharide, which is then conjugated to the purified protein in the presence of EDC (carbodiimide reaction). See Chu et al., Infect. Immun. 1983; 40:245-256; Schneerson et al. Infect. Immun. 1986, 52:519-528. This modification improved the conjugation efficiency and product solubility. The vaccine polyribosylribitol phosphate tetanus protein conjugate (PRP-T) utilizes the improved chemistry to covalently link Hib polysaccharide to tetanus toxoid (see Table 1).
The polyribosylribitol phosphate cross reacting mutant diphtheria toxoid conjugate (PRP-CRM) vaccine, also referred to as Haemophilus b oligosaccharide conjugate (HbOC), does not contain Hib PS. Instead, it utilizes oligosaccharides of about 20 repeat units derived by periodate oxidation of the glycol functionality in the ribitol moiety. The oxidized oligosaccharides are then attached directly to CRM197 a nontoxic mutant form of diphtheria toxin isolated from cultures of Corynebacterium diphtheriae C7 (β197), in the presence of sodium cyanoborohydride (reductive amination). See Anderson et al., J. Immunol, 1989; 142:2464-8; and Anderson, Infect. Immun. 1983, 39:233-238. In this conjugation method, the ratio of oligosaccharide to protein was found to be critical for optimal antibody response. See Kniskern et al., “Conjugation: design, chemistry, and analysis” in Ellis et al., Development and clinical uses of Haemophilus b conjugate vaccines. New York: Marcel Dekker, 1994: 37-69; Anderson et al., J. Immunol. 1989; 142:2464-8.
Compared to the other Hib conjugate vaccines, Hib polysaccharide-Neisseria meningitidis outer membrane protein complex conjugate vaccine (PRP-OMPC) has a number of unique properties. The protein carrier is not a component of the diphtheria, tetanus, and pertussis (DTP) vaccine, but consists of lipopolysaccharide-depleted meningococcal outer membrane vesicles to which are attached size-reduced Hib PS through a thioether linkage. See Marburg et al., J. Amer. Chem. Soc. 1986; 108:5282-5287; Kniskern et al., “Conjugation: design, chemistry, and analysis” in Ellis et al., Development and clinical uses of Haemophilus b conjugate vaccines. New York: Marcel Dekker, 1994: 37-69; Anderson et al., J. Immunol. 1989; 142:2464-8. In this process, separate linkers are attached to both the protein and Hib polysaccharide, followed by fusion of the linkers to form a thioether linkage.
Neisseria meningitides is a leading cause of bacterial meningitis and sepsis throughout the world. Pathogenic meningococci are enveloped by a polysaccharide capsule that is attached to the outer membrane surface of the organism. Thirteen different serogroups of meningococci have been identified on the basis of the immunological specificity of the capsular polysaccharide (Frasch, C.E., et al., “Serotype antigens of Neisseria meningitides and a proposed scheme for designation of serotypes,” Rev Infect Dis. 7(4):504-10, Jul-Aug 1985). Of these thirteen serogroups, five cause the majority of meningococcal disease; these include serogroups A, B, C, W135, and Y. Serogroup A is responsible for most epidemic disease. Serogroups B, C, and Y cause the majority of endemic disease and localized outbreaks. Host defense of invasive meningococci is dependent upon complement-mediated bacteriolysis. The serum antibodies that are responsible for complement-mediated bacteriolysis are directed in large part against the outer capsular polysaccharide.
Conventional vaccines based on meningococcal polysaccharide elicit an immune response against the capsular polysaccharide. These antibodies are capable of complement-mediated bacteriolysis of the serogroup specific meningococci. The meningococcal polysaccharide vaccines were shown to be efficacious in children and adults. However, efficacy was limited in infants and young children, and subsequent doses of the polysaccharide in younger populations elicited a weak or no booster response.
There are a number of approaches that have been employed for activation of the meningococcal PS and for conjugation, as summarized in Table 2. Each mode of activation has the potential to alter important epitopes, even when relatively few sites are activated on the PS molecule. Periodate activation of the group C meningococcal PS, for example, results in chain breakage generating smaller saccharide units with terminal aldehyde groups that can be linked to the protein via reductive amination. Richmond et al., J. Infect. Dis. 1999; 179:1569-72.
TABLE 2Used MethodSaccharide sizeCarrier proteinSpacerProcedurein humans#1ReducedTetanusNoneAldehyde form of PSNoReductivetoxoidcombined with proteinaminationin presence of sodiumcyanoborohydride#2NativeTetanusNonePS and proteinNoCarbodiimidetoxoidcombined in presenceof carbodiimide, thenblocked withethanolamine#3OligosaccharideCRM 197AdipicAminated reducingYesActive ester aacidterminus of theoligosaccharideconjugated to proteinby adipic acid (NHS)2#4ReducedCRM 197NoneAldehyde form ofYesReductivesaccharide combinedaminationwith protein inpresence of sodiumcyanoborohydride#5De-OAc PS bTetanusNoneAldehyde form of PSYesReductivetoxoidcombined with proteinaminationin presence of sodiumcyanoborohydridea N-hydroxysuccinimide diester of adipic acidb Deacetylylated PS only reported for Meningococcal group C
Initial studies on production and optimization of meningococcal group C conjugates were reported well before commercialization of the Hib conjugates. See Beuvery et al., Infect. Immun. 1982; 37:15-22; Beuvery et al., Infect. Immun. 1983; 40:39-45; Beuvery et al., J. Infect. 1983; 6:247-55; Jennings, et al., J. Immunol. 1981; 127:1011-8.
Two different conjugation methodologies have been reported for chemically linking the group C PS to a protein carrier. See Jennings et al., J. Immunol. 1981; 127:1011-8; Beuvery et al., Infect. Immun. 1983; 40:39-45. The first approach employs partially depolymerized PS, which is activated by creation of terminal aldehyde groups through periodate oxidation (Method #1 in Table 2). The aldehydes are then reacted through reductive amination combined with free amino groups on the protein, mostly lysines, in the presence of sodium cyanoborohydride. See Jennings et al., J Immunol 1981; 127:1011-8. In this method, activation occurs at one specific site on the group C PS.
The second approach utilizes the carbodiimide reaction (Method #2 in Table 2) to covalently link carboxylic groups in the high molecular weight PS to lysine ε-amino groups on the carrier protein. The activation sites in this method are more random, compared to periodate activation.
Group C meningococcal conjugates prepared by these two methods have been evaluated in animals. See Beuvery et al., Dev. Biol. Stand. 1986; 65:197-204; and Beuvery et al., J. Infect. 1983; 6:247-55. The conjugates stimulated both T cell independent and T cell dependent responses upon initial immunization. See Beuvery et al., J. Infect. 1983; 6:247-55. Studies have shown that the PS must, however, be covalently linked to the carrier protein to induce a T cell dependent antibody response.
The first group A and group C meningococcal conjugates to be used in clinical trials were prepared by Chiron Vaccines and were reported in 1992 (Method #3 in Table 2). See Costantino et al., Vaccine 1992; 10:691-8. The conjugation method was based upon selective terminal group activation of small oligosaccharides produced by mild acid hydrolysis followed by coupling to a protein through a hydrocarbon spacer. The non-toxic mutant of diphtheria toxin, CRM197, was used as the protein carrier. To activate the oligosaccharides for conjugation, an amino group was added to the end of the oligosaccharide, and then reacted with the N-hydroxysuccinimide diester of adipic acid to create an active ester. This active ester was then covalently bound to lysine ε-amino groups in the CRM197 protein, creating the conjugate.